DOCSLIB.ORG

Downloaded 10/02/21 05:19 AM UTC

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Downloaded 10/02/21 05:19 AM UTC AUGUST 2006 T O URNADRE 1131 Improved Level-3 Oceanic Rainfall Retrieval from Dual-Frequency Spaceborne Radar Altimeter Systems J. TOURNADRE Département d’Océanographie Physique et Spatiale, IFREMER, Plouzané, France (Manuscript received 15 March 2005, in final form 28 November 2005) ABSTRACT Since the launch of the Ocean Topography Experiment (TOPEX)/Poseidon in 1992, several studies have demonstrated that dual-frequency altimeter measurements cannot only accurately detect rain events but can also be used to infer quantitative values. The main problems with these techniques are the limited time and space sampling of nadir-looking instruments and the uncertainty in the height of the freezing level necessary to infer the surface rain rate from the measured signal attenuation. In addition to radar altimeters, altimetric satellites carry microwave radiometers designed to correct for atmospheric water effects. Using a radiative transfer model and simplified rainy atmospheres, a method of inversion of the microwave brightness temperatures in terms of freezing level is presented. The surface rain rate is then computed from the altimeter attenuation and the radiometer freezing level. The rain climatology is computed for the three altimeters currently in operation using a mixed lognormal distribution. Comparison with the Global Pre- cipitation Climatology Project and Special Sensor Microwave Imager (SSM/I) climatologies shows that the use of freezing level greatly improves the altimeter climatology, which is of the same quality as that of the SSM/I for annual mean. The merging of the three altimeters is investigated. The resulting monthly mean rain rates are comparable to those derived from SSM/I. The high along-track resolution of altimeters also allows the determination of the length of rain events. The mean length is close to the SSM/I footprint size in the Tropics, but at higher latitude 80% of the rain has length scales smaller than 10 km, which might explain the relative underestimation of the mean rain rate by SSM/I. 1. Introduction the limited time and space sampling and the other is the uncertainty in the height of the melting layer. The melt- The determination of accurate rain-rate estimates ing-layer height, necessary to infer the surface rain rate over the global ocean has been a challenging task for from the altimeter attenuation that is integrated over over a decade. In situ rain measurements at sea are the atmospheric path, was fixed either to a constant difficult and not always reliable. Because rain is by its value (Chen et al. 1997, 2003) or to a climatological very essence a transitory phenomenon, only satellite- mean, varying with latitude and season (Quartly et al. borne instruments can provide appropriate coverage. 1999). As shown by Quartly et al. (1999), these approxi- Since the launch of the Ocean Topography Experiment mations can lead to large errors in the surface rain-rate (TOPEX)/Poseidon, the first dual-frequency altimeter estimates, especially outside of the tropical regions. to be flown in space, it has been demonstrated by vari- However, altimetric mission satellites carry, in addi- ous studies (Tournadre and Morland 1997; Quartly et tion to radar altimeters, microwave radiometers de- al. 1999; Chen et al. 1997, 2003; Cailliau and Zlotnicki signed primarily to correct the altimetric measurements 2000; McMillan et al. 2002) that this kind of instrument for the effects of atmospheric water (path delay, attenu- cannot only accurately detect rain events but also can ation, etc.). These radiometers measure brightness tem- be used to yield quantitative values. However, there are peratures (Tbs) at two or three frequencies between 18 still two main problems with these techniques—one is and 37 GHz. These frequencies are close to the ones used by the Special Sensor Microwave Imager (SSM/I) for which several algorithms have been developed to Corresponding author address: J. Tournadre, IFREMER, infer both freezing-level altitude and rain rate (Wilheit Technopole Brest-Iroise, 29280 Plouzane´, France. et al. 1977, 1991). In this study, we present a method E-mail: [email protected] that combines altimeter and radiometer data to simul- © 2006 American Meteorological Society Unauthenticated | Downloaded 10/02/21 05:19 AM UTC JTECH1897 1132 JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY VOLUME 23 taneously estimate the melting-layer altitude and rain 1) TOPEX/POSEIDON rate. The TOPEX/Poseidon satellite was developed by the Currently, three altimeter missions are in operation, National Aeronautics and Space Administration Jason TOPEX/Poseidon, , and the Environmental Sat- (NASA) and the French Space Agency [Centre Na- ellite (Envisat). These satellites have different charac- tional d’Etudes Spatiales (CNES)]. It was launched on teristics and orbits, and thus different time and space 10 August 1992, and up to now it has provided more samplings of the global ocean. The simultaneous avail- than 12 yr of data. TOPEX/Poseidon is dedicated to ability of these three data sources gives a good oppor- ocean altimetry, and its orbit and satellite bus and pay- tunity to investigate the potential of multisatellite rain load were optimized to map the ocean surface. The climatology. The data merging requires the intercom- satellite carries two altimeters, one developed by CNES parison and the intercalibration of the freezing-level and the other by NASA. The Poseidon-1 altimeter de- and the rain-rate estimates from the different satellite veloped by CNES is a single-frequency, solid-state ex- systems. perimental instrument intended to demonstrate new A key advantage of the altimeter systems is their high technology, and it operates approximately 10% of the along-track resolution (ϳ6 km), which allows the study time. The NASA radar altimeter (NRA), which oper- of the length scales of the rain events over the global ates at 13.6 (Ku band) and 5.3 (C band) GHz simulta- ocean. The rain cell size distribution and its geographi- neously, is the primary sensor of the mission. Only the cal variability are significant and informative character- dual-frequency NRA data are considered in this study. istics for the description of rain fields (Sauvageot et al. Depending on the sea state, the altimeter footprint var- 1999). The information obtained from altimeters would ies from 5- to 10-km radius. The satellite samples the complement those obtained on land using meteorologi- ocean surface between 66°S and 66°N at a 1-s interval cal radars. The joint analysis of length scales and radi- (corresponding to a 5.8 km ground distance) for each of ometer measurements can also give insight into the ef- the 254 passes that make up a 9.9156-day repeat cycle. fect of partial beam filling and explain the differences A detailed description of the NRA instrument and data between rain estimates from the altimeter and radiom- processing is given in Zieger et al. (1991) and Marth et eter. al. (1993). In section 2, the three altimeter missions currently The satellite also carries the TOPEX/Poseidon mi- available, that is, TOPEX/Poseidon, Jason, and En- crowave radiometer (TMR), which was primarily de- visat, and the auxiliary data used for validation are signed to provide corrections for the altimeter range briefly described. In section 3, the method to infer the errors (the so-called path delay) induced by the highly freezing level and the rain rate is presented. The freez- variable atmospheric water vapor content (Ruf et al. ing-level estimates are validated by comparison to 1994; Janssen et al. 1995; Keihm et al. 1995). TMR is a those of the National Centers for Environmental Pre- modified version of the Special Sensor Microwave Ra- diction (NCEP) NWP model and SSM/I in section 4. diometer (SMMR) that flew on Seasat and Nimbus-7.It Section 5 presents the mean rain fields derived from the operates at three frequencies of 18, 21, and 37 GHz in three altimeters and also presents a comparison with a nadir-viewing direction with a footprint diameter of the ones estimated from SMM/I and the Global Pre- 43.4, 36.4, and 22.9 km, respectively (Ruf et al. 1994). cipitation Climatology Project (GPCP). The merging TMR is temporally and spatially coaligned with the of the three altimeters is also tackled in this section. TOPEX/Poseidon altimeter in order to accommodate The analysis of the distribution of the rain cell length its mission requirements. In addition to water vapor and its influence on the mean rain rate is presented in retrieval, TMR also provides useful data of cloud liquid section 6. water. 2) JASON 2. Data The CNES–NASA Jason mission is designed to en- sure the continuity of the observation and monitoring a. Dual-frequency altimeter missions: of the ocean provided by TOPEX/Poseidon and it has TOPEX/Poseidon, Jason, and Envisat basically the same characteristics. It was launched on 7 The data from the archive of the three altimeter mis- December 2001. Its main instrument is the Poseidon-2 sions, which use dual-frequency radar altimeter, have altimeter, which is derived from the experimental been used in this study. The instruments and data are Poseidon-1 altimeter. It is a compact, low-power, low- briefly presented in this section. mass instrument offering a high degree of reliability. Unauthenticated | Downloaded 10/02/21 05:19 AM UTC AUGUST 2006 T O URNADRE 1133 The dual-frequency Poseidon-2 operates at the same level is flagged as missing. If multiple zero crossings are frequencies as the NRA, that is, at 13.6 (Ku band) and found, resulting from temperature inversions, only the 5.3 (C band) GHz. A detailed description of the Posei- lowest Z0 value is kept. don-2 altimeter is given in Ménard and Fu (2001). The Jason microwave radiometer (JMR), a Jet Pro- 2) RAIN CLIMATOLOGY pulsion Laboratory instrument of TMR heritage, is a Two types of rain climatology are used for compari- passive receiver that collects radiation emitted by the son in this study.
Load more

Recommended publications
  • Aviation Weather Services (AC 00-45H)
    U.S. Department Advisory of Transportation Federal Aviation Administration Circular Subject: Aviation Weather Services Date: 1/8/18 AC No: 00-45H Initiated by: AFS-400 Change: 1 1 PURPOSE OF THIS ADVISORY CIRCULAR (AC). This AC explains U.S. aviation weather products and services. It provides details when necessary for interpretation and to aid usage. This publication supplements its companion manual, AC 00-6, Aviation Weather, which documents weather theory and its application to aviation. The objective of this AC is to bring the pilot and operator up-to-date on new and evolving weather information and capabilities to help plan a safe and efficient flight, while also describing the traditional weather products that remain. 2 PRINCIPAL CHANGES. This change adds guidance and information on Graphical Forecast for Aviation (GFA), Localized Aviation Model Output Statistics (MOS) Program (LAMP), Terminal Convective Forecast (TCF), Polar Orbiting Environment Satellites (POES), Low-Level Wind Shear Alerting System (LLWAS), and Flight Path Tool graphics. It also updates guidance and information on Direct User Access Terminal Service (DUATS II), Telephone Information Briefing Service (TIBS), and Terminal Doppler Weather Radar (TDWR). This change removes information regarding Area Forecasts (FA) for the Continental United States (CONUS). 1/8/18 AC 00-45H CHG 1 PAGE CONTROL CHART Remove Pages Dated Insert Pages Dated Pages iii thru xii 11/14/16 Pages iii thru xiii 1/8/18 Pages 1-8 and 1-9 11/14/16 Page 1-8 1/8/18 Page 3-57 11/14/16 Pages 3-57 thru
    [Show full text]
  • On the Use of Radiosondes in Freezing Precipitation
    MARCH 2018 W AUGH AND SCHUUR 459 On the Use of Radiosondes in Freezing Precipitation SEAN WAUGH NOAA/OAR/National Severe Storms Laboratory, and Cooperative Institute for Mesoscale Meteorological Studies, University of Oklahoma, Norman, Oklahoma TERRY J. SCHUUR Cooperative Institute for Mesoscale Meteorological Studies, University of Oklahoma, and NOAA/OAR/National Severe Storms Laboratory, Norman, Oklahoma (Manuscript received 13 April 2017, in final form 4 January 2018) ABSTRACT Radiosonde observations are used the world over to provide critical upper-air observations of the lower atmosphere. These observations are susceptible to errors that must be mitigated or avoided when identified. One source of error not previously addressed is radiosonde icing in winter storms, which can affect forecasts, warning operations, and model initialization. Under certain conditions, ice can form on the radiosonde, leading to decreased response times and incorrect readings. Evidence of radiosonde icing is presented for a winter storm event in Norman, Oklahoma, on 24 November 2013. A special sounding that included a particle imager probe and a GoPro camera was flown into the system producing ice pellets. While the iced-over temperature sensor showed no evidence of an elevated melting layer (ML), complementary Particle Size, Image, and Velocity (PASIV) probe and polarimetric radar observations provide clear evidence that an ML was indeed present. Radiosonde icing can occur while passing through a layer of supercooled drops, such as frequently found in a subfreezing layer that often lies below the ML in winter storms. Events that have warmer/deeper MLs would likely melt any ice present off the radiosonde, minimizing radiosonde icing and allowing the ML to be detected.
    [Show full text]
  • Detecting the Melting Layer with a Micro Rain Radar Using a Neural
    https://doi.org/10.5194/amt-2019-248 Preprint. Discussion started: 23 September 2019 c Author(s) 2019. CC BY 4.0 License. Detecting the Melting Layer with a Micro Rain Radar Using a Neural Network Approach Maren Brast1 and Piet Markmann1 1METEK Meteorologische Messtechnik GmbH, Fritz-Straßmann-Str. 4, 25337 Elmshorn, Germany Correspondence: Piet Markmann ([email protected]) Abstract. A new method using the Micro Rain Radar (MRR) to determine the melting layer height is presented. The MRR is a small vertically pointing frequency modulated continuous wave radar which measures Doppler spectra of precipitation. From these Doppler spectra, various variables such as Doppler velocity or spectral width can be derived. The melting layer is visible through a higher reflectivity and an acceleration of the falling particles, among others. These characteristics are fed to a neural 5 network to determine the melting layer height. For the training of the neural network, the melting layer height is determined manually. The neural network is trained and tested using data from two sites covering all seasons. For most cases, it is well able to detect the correct melting layer height. Sensitivity studies show that the neural network is able to handle different settings of the MRR. Comparisons to radiosonde data and cloud radar data show a good agreement in melting layer heights. 10 1 Introduction The bright band in radar meteorology shows the location of the layer where solid precipitation melts into rain in a complex process. Near the freezing level, aggregation leads to larger ice particles. When melting begins, the particles are coated by liquid water, appearing as very large raindrops, until all ice is melted and the particles collapse into raindrops.
    [Show full text]
  • In the Troposphere, the Temperature Decreases with Altitude
    Indian Journal of Geo-Marine Sciences Vol. 44(7), July 2015, pp. 1071-1095 Study of TRMM estimated freezing level height in the 36N – 36S region Rajasri Sen Jaiswal *, Sonia R Fredrick, M Rasheed, V S Neela & Leena Zaveri Centre for Study on Rainfall and Radio wave Propagation, Sona College of Technology, Salem-636005, India [E-mail: [email protected] ] Received 21 April 2014; revised 14 October 2014 In this paper the freezing level height (HFL), a very important parameter in cloud physics has been studied in the latitudinal belt 36N-36S, during the period 1999-2002 and 2007-2008. Present study shows that the latitude is the major controlling factor of the HFL, rather than LH or temperature. Further, it is seen that the HFL depends on season. The study also reveals that the HFL responds in a different manner over the continental and the oceanic sites. Functional relationship between the HFL and latitude shows seasonal dependence. It also presents the range of the HFL, its maximum and minimum values and the months of occurrence of the maxima/minima over few selected locations, including India. [Keywords: Freezing level height, latitude, temperature, latent heat] Introduction In the troposphere, the temperature decreases percentage of liquid precipitation8. This is shown with altitude. The height at which the temperature for both mid latitude and Arctic region. Freezing falls to 0°C is known as 0°C isotherm height, or level height is a very important parameter in freezing level1. The freezing level during rainy climatology. During the precipitation process, condition is known as the rain height.
    [Show full text]
  • NOAA Technical Memorandum NWS WR-215 WEATHERTOOLS Tom
    NOAA Technical Memorandum NWS WR-215 WEATHERTOOLS Tom Egger National Weather Service Weather Service Forecast Office Boise, Idaho " . f. October 1991 u.s. DEPARTMENT OF I National Oceanic and National Weather COMMERCE Atmospheric Administration I Service 75 A Study of the Low Level Jet Stream of the San Joaquin Valley. Ronald A. Willis and NOAA TECHNICAL MEMORANDA Philip Williams, Jr., May 1972. (COM 72 10707) ..~ _ Hot~ National Weather Service, Western Region Subseries 76 Monthly Climatological Charts of the Behavior of Fog and Low Stratus at Los Angeles International Airport. Donald M. Gales, July 1972. (COM 72 11140) Th National Weather Service (NWS) Western Region (WR) Subseries provides an informal 77 A Study of Radar Echo Distribution in Arizona During July and August. John E. Hales, Jr., me~ium for the documen?ttion and qui.ck ~isseminntion of results not. appropriate, or not .yet July 1972. (COM 72 11136) . lv for formal publication. The senes ts used to report on work m progress, to describe 78 Forecasting Precipitation at Bakersfield, Cahforma, Usmg Pressure Grnd1ent Vectors. Earl ~~~h0ica1 procedures and practices! or. to relate progre_ss to. a limited. audience. These Technical T. Riddiough, July 1972. (COM 72 11146) Memoranda will report on investtgatlons ~evoted pru_nanly _to !egional and local problems of 79 Climate of Stockton, California. Robert C. Nelson, July 1972. (COM 72 10920) interest mainly to personnel, and hence wdl not be Widely dtstrtbuted. 80 Estimation of Number of Days Above or Below Selected Temperatures. Clarence M. Sakamoto, October 1972. (COM 72 10021) Papers 1 to 25 are in the former series, ESSA Technical Memoranda, Western Region Technical 81 An Aid for Forecasting Summer Maximum Temperatures at Seattle, Washington.
    [Show full text]
  • Models and Characteristics of Freezing Rain and Freezing Drizzle for Aircraft Icing Applications
    DOT/FAA/AR-09/45 Models and Characteristics of Air Traffic Organization NextGen & Operations Planning Freezing Rain and Freezing Office of Research and Technology Development Drizzle for Aircraft Icing Washington, DC 20591 Applications Richard K. Jeck January 2010 Final Report This document is available to the U.S. public through the National Technical Information Services (NTIS), Springfield, Virginia 22161. U.S. Department of Transportation Federal Aviation Administration NOTICE This document is disseminated under the sponsorship of the U.S. Department of Transportation in the interest of information exchange. The United States Government assumes no liability for the contents or use thereof. The United States Government does not endorse products or manufacturers. Trade or manufacturer's names appear herein solely because they are considered essential to the objective of this report. This document does not constitute FAA certification policy. Consult your local FAA aircraft certification office as to its use. This report is available at the Federal Aviation Administration William J. Hughes Technical Center’s Full-Text Technical Reports page: actlibrary.act.faa.gov in Adobe Acrobat portable document format (PDF). Technical Report Documentation Page 1. Report No. 2. Government Accession No. 3. Recipient's Catalog No. DOT/FAA/AR-09/45 4. Title and Subtitle 5. Report Date MODELS AND CHARACTERISTICS OF FREEZING RAIN AND FREEZING January 2010 DRIZZLE FOR AIRCRAFT ICING APPLICATIONS 6. Performing Organization Code 7. Author(s) 8. Performing Organization Report No. Richard K. Jeck 9. Performing Organization Name and Address 10. Work Unit No. (TRAIS) Federal Aviation Administration William J. Hughes Technical Center Airport and Aircraft Safety R&D Group Flight Safety Team 11.
    [Show full text]
  • V12.6 Radar Legends (When from Internet)
    Weather Legends in FOREFLIGHT MOBILE 17th Edition Covers ForeFlight Mobile v12.6 Radar Legends (when from Internet) Snowy/Icy Precipitation Mixed Precipitation Rain Echo top (in 100’s of feet) ex: 24,900’ Storm Track Estimated position in 20, 40 and 60 minutes ForeFlight Mobile Legends v12.6 and later! 2 Rain - Radar Intensity (dBZ) vs. Color Based on RGB values assigned to dBZ range(s) dBZ Internet Color1 ADS-B Color2,4 SiriusXM Color3 5 none shown 10 none shown 15 20 25 30 35 40 45 50 55 60 65 70 75 95 1. Colors are interpolated between levels when rendered on an image. 2. ADS-B (FIS-B) NEXRAD radar is displayed with 6 intensity ranges. 3. Some dBZ intensity/color divisions do not fall exactly on 5 dBZ lines, so are shown as close as possible to specification. ForeFlight Mobile Legends v12.6 and later! 3 Mixed Rain/Snow - Radar Intensity (dBZ) vs. Color Based on RGB values assigned to dBZ range(s) dBZ Internet Color1 ADS-B Color2,3,5 SiriusXM Color4 5 none shown 10 none shown 15 20 25 30 35 40 45 50 55 60 65 70 75 1. Colors are interpolated between levels when rendered on an image. 2. ADS-B (ie: FIS-B) NEXRAD radar is displayed with 6 intensity ranges. 3. FIS-B NEXRAD doesn't include precipitation type, so "Mixed" is displayed at the same reflectivity colors as rain. See AIM Chapter 7: http://tfmlearning.fly.faa.gov/ publications/atpubs/aim/chap7/aim0701.html 4. Some dBZ intensity/color divisions do not fall exactly on 5 dBZ lines, so are shown as close as possible to specification.
    [Show full text]
  • General Aviation Pilot's Guide to Preflight Weather
    Federal Aviation Administration General Aviation Pilot’s Guide to Preflight Weather Planning, Weather Self-Briefings, and Weather Decision Making General Aviation Pilot’s Guide Preflight Planning, Weather Self-Briefings, and Weather Decision Making Foreword…………………………………………………………… ii Introduction……………………………………………………… 1 I Preflight Weather Planning…………………………………… 2 Perceive – Understanding Weather Information…………………..... 2 Process – Analyzing Weather Information ………………..………... 7 Perform – Making A Weather Plan.…………………………………... 11 II In-flight Weather Decision-Making…………………………… 14 Perceive – In-flight Weather Information…………………….......... 14 Process – (Honestly) Evaluating In-flight Conditions…………….. 16 Perform – Putting It All Together…………………………………… 20 III Post-Flight Weather Review…………………………………... 22 IV Resources………………………………………………………… 23 Appendix 1 – Weather Products & Providers Chart……………... 24 Appendix 2 – Items for Standard Briefing…………………………. 25 Appendix 3 – Automated Weather Systems (definitions)………... 26 Appendix 4 – Developing Personal Weather Minimums. ……….. 27 Appendix 5 –Aviation Weather Analysis Worksheets…..…………. 31 Appendix 6 –Weather Analysis Checklists (VFR)…………………. 32 Appendix 7 – Weather Analysis Checklists (IFR)………………….. 34 Appendix 8 – Estimating In-flight Visibility & Cloud Clearance…….. 36 Preflight Guide v. 1.1 Foreword This guide is intended to help general aviation (GA) pilots, especially those with relatively little weather-flying experience, develop skills in obtaining appropriate weather information, interpreting
    [Show full text]
  • Student Study Notes – Canadian PPL Aviation Ground School: Weather & Meteorology
    Student Study Notes – Canadian PPL Aviation Ground School: Weather & Meteorology This version of my “Weather & Meteorology” study notes is from January 1st, 2017. I’ll update this document any time I find the need to make any changes, and as I continue to progress through additional training. I am sharing these study notes for anyone else who is taking their PPL in Canada. These aren’t intended as a replacement for proper training. I’m only sharing these notes as a supplement covering many of the key points that I decided that I really needed to memorize while going through my own PPL studies. The info in these notes comes from a large number of different sources: The Transport Canada Flight Training Manual, Transport Canada’s Aeronautical Information Manual (AIM), various flight schools and instructors (in multiple provinces), and numerous other books and online sources. These notes are not always in any particular order, although I tried to keep similar topics together in many cases. Please note that while I have made every effort to ensure that all of the information in these notes is accurate, based on the sources from which I learned, you should verify everything here against what you’ve learned in your own study programs. I (Jonathan Clark) shall not assume any liability for errors or omissions in these notes, and your official pilot training should always supersede any information presented herein. As the Canadian PPL curriculum is updated occasionally, I recommend that if you want to be 100% certain that everything in this set of study notes is correct, you should print a copy and ask your instructor to review these notes with you.
    [Show full text]
  • 1 Comparison of Lower-Tropospheric Temperature Climatologies And
    Comparison of Lower-Tropospheric Temperature Climatologies and Trends at Low and High Elevation Radiosonde Sites Dian J. Seidel and Melissa Free Air Resources Laboratory (R/ARL) National Oceanic and Atmospheric Administration 1315 East West Highway Silver Spring, Maryland, USA [email protected] phone: 301-713-0295 ext. 126 fax: 301-713-0119 Submitted to Climatic Change Special Issue on Climate Change at High Elevation Sites November 27, 2001 Revised: June 17, 2002 1 2 Abstract Observations of rapid retreat of tropical mountain glaciers over the past two decades seem superficially at odds with observations of little or no warming of the tropical lower troposphere during this period. To better understand the nature of temperature and atmospheric freezing level variability in mountain regions, on seasonal to multidecadal time scales, this paper examines long-term surface and upper-air temperature observations from a global network of 26 pairs of radiosonde stations. Temperature data from high and low elevation stations are compared at four levels: the surface, the elevation of the mountain station surface, 1 km above the mountain station, and 2 km above the mountain station. Climatological temperature differences between mountain and low elevation sites show diurnal and seasonal structure, as well as latitudinal and elevational differences. Atmospheric freezing-level heights tend to decrease with increasing latitude, although maximum heights are found well north of the equator, over the Tibetan Plateau. Correlations of interannual anomalies of temperature between paired high and low elevation sites are relatively high at 1 or 2 km above the mountain station. But at the elevation of the station, or at the two surface elevations, correlations are lower, indicating decoupling of the boundary layer air from the free troposphere.
    [Show full text]
  • A Matter of Wet-Bulb Temperature
    Examensarbete vid institutionen för geovetenskaper ISSN 1650-6553 Nr 48 Snow or rain? - A matter of wet-bulb temperature Arvid Olsen Abstract Accurate precipitation-type forecasts are essential in many areas of our modern society and therefore there is a need to develop proper working methods for this purpose. Focus of this work has been to study important physical processes decisive in deciding both the temperature of the precipitation particles, hence affecting their phase, and the surrounding air. Two major latent heating effects have been emphasized, melting effect and cooling by evaporation/sublimation. Melting of the snow flakes subtracts heat from the surroundings and hence acts as a cooling agent. Phase transformation from solid/liquid into the gas phase also needs heat which here results in a cooling tendency. These two mechanisms may sometimes have a crucial influence for deciding the correct precipitation-type. The melting effect is discussed in a paper about a snow event in Tennessee in USA, and another paper describing an event in Japan showing the influence of the evaporation/sublimation process. In the latter case the wet-bulb temperature, Tiw as a physical correct discriminator between snow and rain is obtained. A numerical weather prediction model (HIRLAM) is being used to study different condensation schemes during three weather situations occurring in Sweden. These are Rasch/Kristjánsson condensation scheme, Sundqvist original condensation scheme and a modification of the latter scheme. In the modified Sundqvist condensation scheme the Tiw has been implemented as a limit temperature between snow and rain. The results are showing differences between the two main schemes concerning the total precipitation (both snow and rain).
    [Show full text]
  • Detection of the Freezing Level with Polarimetric Weather Radar
    https://doi.org/10.5194/amt-2020-375 Preprint. Discussion started: 28 September 2020 c Author(s) 2020. CC BY 4.0 License. Detection of the freezing level with polarimetric weather radar Daniel Sanchez-Rivas1,* and Miguel A Rico-Ramirez1,* 1Department of Civil Engineering, University of Bristol, Bristol, BS8 1TR, United Kingdom Correspondence: Daniel Sanchez-Rivas ([email protected]) Abstract. Accurate estimation of the Freezing Level (FL) is essential in radar rainfall estimation to mitigate the bright band enhancement, to classify hydrometeors, to correct for rain-attenuation and to calibrate radar measurements. Here we present a novel and robust FL estimation algorithm that can be applied to either Vertical Profiles (VPs) or Quasi-Vertical Profiles (QVPs) built from operational polarimetric weather radar scans. The algorithm depends only on data collected by the radar itself, 5 and it is based on the detection of strong gradients within the profiles and relies on the combination of several polarimetric variables. The VPs and QVPs of Z showed a good similarity in the profiles (r 0.7) even though the QVPs are built from H ≈ low-elevation angles. The algorithm is applied to one year of rainfall events and validated using measured FLs from radiosonde data. The results demonstrated that combining the profiles of ZH , ρHV and the gradient of the velocity V showed the best FL estimation performance when using VPs, whereas combining the profiles of ZH , ρHV and ZDR showed the best FL estimation 10 performance when using QVPs. The VP computed from the gradient of the velocity showed to be extremely valuable in the FL estimation when using VPs.
    [Show full text]